Method of operating a three-phase slurry reactor

ABSTRACT

A method of operating a three-phase slurry reactor includes feeding at a low level at least one gaseous reactant into a vertically extending slurry body of solid particles suspended in a suspension liquid, the slurry body being contained in a plurality of vertically extending horizontally spaced slurry channels inside a common reactor shell, the slurry channels being defined between vertically extending horizontally spaced divider walls or plates and each slurry channel having a height, width and breadth such that the height and breadth are much larger than the width. The gaseous reactant is allowed to react as it passes upwardly through the slurry body present in the slurry channels, thereby to form non-gaseous and/or gaseous product. Gaseous product and/or unreacted gaseous reactant is allowed to disengage from the slurry body in a head space above the slurry body.

THIS INVENTION relates to a method of operating a three-phase slurryreactor and to a three-phase slurry reactor.

Considerable risk is encountered when technology is scaled up from pilotplant scale to commercial plant scale in order to reap the benefits ofeconomy of scale. Three-phase slurry reactors typically exhibitscale-dependent macro-mixing effects and the aforementioned risk is thusapplicable when three-phase slurry reactors are scaled up. It will thusbe an advantage if a method can be found which can significantly reducethe risk associated with upscaling of three-phase slurry reactors. Inaddition, reactor designs in which the mixing patterns inside thereactor can be more readily modelled or predicted from experimentationhave the benefit that the extent of usually undesirable back-mixing canbe limited thereby potentially allowing an optimal combination ofdesirable plug-flow characteristics (usually good productivity and goodselectivity) and well-mixed characteristics (often required fordesirable solids distribution and even temperature profiles).

A solution that has been proposed is to create zones in the reactor thateffectively mimic the behaviour of a reactor with a smallercharacteristic diameter. In this manner the behaviour of the large scalereactor can be predicted to some extent, since it effectively consistsof the sum of a number of smaller reactors of effectively pilot plantscale. However, one is still largely dependent on working within thebounds of the macro-mixing patterns that are established in the reactorwith a smaller characteristic diameter. It would thus be an advantage ifa method can be found that allows designers additional degrees offreedom to, at least to some extent, control the mixing patterns thatare established in a three-phase slurry reactor.

Three-phase slurry reactors are commonly used for highly exothermicreactions due to their excellent heat removal characteristics. However,with the introduction of ever more active catalysts and more intensiveuse of reactor volume, even the heat removal ability of three-phaseslurry reactors is being tested.

In light of what has been said before, it will thus be an advantage if amethod can be found which significantly reduces the risk associated withupscaling of three-phase slurry reactors by allowing the designeradditional degrees of freedom to exert some control over the mixingpatterns in the reactor, while simultaneously increasing the heatremoval ability of the reactor.

According to one aspect of the invention, there is provided a method ofoperating a three-phase slurry reactor, the method including

-   feeding at a low level at least one gaseous reactant into a    vertically extending slurry body of solid particles suspended in a    suspension liquid, the slurry body being contained in a plurality of    vertically extending horizontally spaced slurry channels inside a    common reactor shell, the slurry channels being defined between    vertically extending horizontally spaced divider walls or plates and    each slurry channel having a height, width and breadth such that the    height and breadth are much larger than the width;-   allowing the gaseous reactant to react as it passes upwardly through    the slurry body present in the slurry channels, thereby to form    non-gaseous and/or gaseous product;-   allowing gaseous product and/or unreacted gaseous reactant to    disengage from the slurry body in a head space above the slurry    body;-   withdrawing gaseous product and/or unreacted gaseous reactant from    the head space; and-   if necessary, maintaining the slurry body at a desired level by    withdrawing slurry or suspension liquid, including non-gaseous    product if present, or by adding slurry or suspension liquid.

The method may include allowing slurry to pass downwardly from a highlevel in the slurry body to a lower level thereof, using one or moredowncomer zones or downcomers inside the reactor shell.

At least some of the slurry channels may be in slurry flow communicationabove upper ends of the slurry channels.

The divider walls or plates of at least some of the slurry channels mayseparate said slurry channels from adjacent heat transfer medium flowspaces. The method may include passing a heat transfer medium throughthe heat transfer medium flow spaces to exchange heat in indirectrelationship with the slurry body present in the slurry channels.

Heat transfer surfaces of the reactor, such as those of the dividerwalls or plates, may optionally be shaped or textured to increase theirheat transfer surface area or to improve heat transfer coefficientscompared to those of smooth divider walls or plates. The shaping ortexturing may include, amongst other methods known to persons skilled inthe art, the use of dimpled, ribbed or finned walls or plates.

According to a second aspect of the invention, there is provided amethod of operating a three-phase slurry reactor, the method including

-   feeding at a low level at least one gaseous reactant into a    vertically extending slurry body of solid particles suspended in a    suspension liquid, the slurry body being contained in a plurality of    vertically extending horizontally spaced slurry channels inside a    common reactor shell, at least some of the slurry channels being in    slurry flow communication above open upper ends of the slurry    channels and at least some of the slurry channels being defined by    walls separating the slurry channels from a heat transfer medium    flow space or spaces;-   allowing the gaseous reactant to react as it passes upwardly through    the slurry body present in the slurry channels, thereby to form a    non-gaseous and/or a gaseous product;-   passing a heat transfer medium through the heat transfer medium flow    space or spaces to exchange heat in indirect relationship with the    slurry body present in the slurry channels;-   allowing slurry to pass downwardly from a high level in the slurry    body to a lower level thereof, using one or more downcomer zones or    downcomers inside the reactor shell;-   allowing gaseous product and/or unreacted gaseous reactant to    disengage from the slurry body in a head space above the slurry    body;-   withdrawing gaseous product and/or unreacted gaseous reactant from    the head space; and-   if necessary, maintaining the slurry body at a desired level by    withdrawing slurry or suspension liquid, including non-gaseous    product if present, or by adding slurry or suspension liquid.

The slurry channels are preferably isolated from each other betweentheir open upper ends and open lower ends, and are preferably separatedfrom each other by heat transfer medium flow spaces. In other words, themethod preferably includes preventing slurry flow communication at allelevations between the open upper ends and lower ends of the slurrychannels, so that the slurry channels are discrete, defining completelyindividualised reaction chambers.

The slurry channels used in the method according to the second aspect ofthe invention may be defined by vertically extending tubes between tubesheets, with the heat transfer medium flow space being defined betweenthe tube sheets and surrounding the tubes. The tubes typically havediameters of at least about 10 cm.

Instead, the slurry channels may be defined by vertically extendinghorizontally spaced divider walls or plates, with heat transfer mediumflow spaces also being defined between vertically extending horizontallyspaced divider walls or plates, at least some of the slurry channelsbeing separated from adjacent heat transfer medium flow spaces by commonor shared divider walls or plates.

The divider walls or plates may be parallel to each other, definingslurry channels and heat transfer medium flow spaces with a height,width and breadth such that the height and breadth are typically muchlarger than the width. In other words, each divider wall has a heightand a breadth which are substantial, a relatively small thickness and isspaced relatively closely from an adjacent divider wall, therebydefining vertically extending parallelipipedal channels or spaces withone dimension much smaller than the other two dimensions.

Heat transfer surfaces of the reactor, such as those of the dividerwalls or plates or tubes, may optionally be shaped or textured toincrease their heat transfer surface area or to improve heat transfercoefficients compared to those of smooth divider walls or smoothcylindrical tubes. The shaping or texturing may include, amongst othermethods known to persons skilled in the art, the use of dimpled, ribbedor finned walls or plates or tubes.

When the slurry channels are defined by divider walls, the slurry andheat transfer medium may be present in slurry channels and heat transfermedium flow spaces that are arranged alternately. Each slurry channelmay thus be flanked by, or sandwiched between, two heat transfer mediumflow spaces, except possibly for radially outer slurry channels.

The downward flow of slurry in the downcomer zones or downcomers may besufficiently high that there is substantially no downward flow of slurryin the slurry channels.

While it is believed that the method can, at least in principle, havebroader application, it is envisaged that the solid particles willnormally be catalyst particles for catalysing the reaction of thegaseous reactant or gaseous reactants into a liquid product and/or agaseous product. The suspension liquid will normally, but notnecessarily always, be liquid product, with liquid phase or slurry thusbeing withdrawn from the slurry body to maintain the slurry body at adesired level.

Furthermore, while it is also believed that, in principle, the methodcan have broader application, it is envisaged that it will haveparticular application in hydrocarbon synthesis where the gaseousreactants are capable of reacting catalytically exothermically in theslurry body to form liquid hydrocarbon product and, optionally, gaseoushydrocarbon product. In particular, the reaction or hydrocarbonsynthesis may be Fischer-Tropsch synthesis, with the gaseous reactantsbeing in the form of a synthesis gas stream comprising mainly carbonmonoxide and hydrogen, and with both liquid and gaseous hydrocarbonproducts being produced and the heat transfer medium being a coolingmedium, e.g. boiler feed water.

For hydrocarbon synthesis, the slurry channels will typically have aheight of at least 0.5 m, preferably at least 1 m, more preferably atleast 2 m, but may even be 4 m or higher. The slurry channels willtypically have a width of at least 2 cm, preferably at least 3.8 cm,more preferably at least 5 cm. The width of the slurry channels willtypically not exceed 50 cm, more preferably the width will not exceed 25cm, more preferably the width will not exceed 15 cm. The slurry channelswill typically have a breadth in the range of approximately 0.2 m to 1m. The reactor shell will typically have a diameter of at least 1 m,preferably at least 2.5 m, more preferably at least 5 m, but one shouldnote that an object of the invention is to neutralise the effect ofreactor diameter on the reactor behaviour.

As will be appreciated, each slurry channel, whether defined betweendivider walls or defined by a tube, functions independently from thereactor shell and can be configured to function to a large extentindependently from other slurry channels. Design and testing of a singleslurry channel or a small subgroup of slurry channels on a pilot scaleis feasible, with scale-up to a commercial scale reactor comprising aplurality of the slurry channels then becoming quite easy and being lessrisky, provided scale-dependent macro-mixing effects are managedproperly.

Furthermore, when downcomers or downcomer zones are employed with asufficient downward slurry flow such that there is substantially nodownward flow of slurry in the slurry channels, the establishment of amacro-mixing pattern, other than that dictated by the defined downflowand upflow zones, over the reactor is practically impossible.

The method may include cooling the gas from the head space to condenseliquid product, e.g. liquid hydrocarbons and reaction water, separatingthe liquid product from the gases to provide a tail gas, and recyclingat least some of the tail gas to the slurry body as a recycle gasstream.

Vertically extending, horizontally disposed reactor zones may be definedinside the reactor shell, where each horizontally disposed reactor zoneincludes a plurality of slurry channels and optionally one or more heattransfer medium flow spaces. The method may include preventing slurryflow communication between adjacent vertically extending, horizontallydisposed reactor zones and at all elevations between upper and loweropen ends of the slurry channels in a horizontally disposed reactorzone. This may be achieved, for example, by providing the horizontallydisposed reactor zones with vertically extending side walls, or byarranging the divider walls in adjacent horizontally disposed reactorzones at perpendicular angles so that an end divider wall in one of thehorizontally disposed reactor zones in effect forms a side wall for theadjacent horizontally disposed reactor zone.

The method may include containing the slurry body in vertically spacedreactor zones each including a plurality of slurry channels andoptionally one or more heat transfer medium flow spaces. An intermediateslurry zone may be defined between vertically spaced reactor zones.

The method may include feeding at least one gaseous stream into anintermediate zone between two vertically spaced reactor zones. Thegaseous stream may be a recycle gas stream. If desired, the gaseousstream may be fed such that a portion of the cross-sectional area of thereactor is not gassed with the gaseous stream.

One or more of the downcomer zones or downcomers may extend from at orabove the open upper ends of the slurry channels, or slurry channels inan upper vertically spaced reactor zone, to at or below open lower endsof the slurry channels, or slurry channels in a bottom vertically spacedreactor zone.

Instead, one or more of the downcomer zones or downcomers may extendfrom at or above the open upper ends of the slurry channels of avertically spaced reactor zone, to at or below open lower ends of theslurry channels of said vertically spaced reactor zone, often into anintermediate zone below said vertically spaced reactor zone. A lower orhigher vertically spaced reactor zone may include a similar downcomerzone or downcomer, which may be staggered in plan view from thedowncomer zone or downcomer in the vertically spaced reactor zone aboveor below, or which may be in register with the downcomer zone ordowncomer in the vertically spaced reactor zone above or below.

If desired, a downcomer zone may include a heat transfer medium flowspace or spaces, and/or a filter to separate solid particles from thesuspension liquid.

Allowing slurry to pass downwardly in a downcomer zone or downcomer mayinclude preventing or inhibiting gaseous reactant or reactants fromentering the downcomer zone, e.g. by providing a baffle, and/or it mayinclude degassing the slurry in the downcomer zone or downcomer, e.g. byproviding a degasser at an upper end of the downcomer zone or downcomer.

The method may include allowing slurry flow communication betweenhorizontally disposed reactor zones in one or more of the intermediatezones, and/or in the bottom of the reactor below the open lower ends ofslurry channels.

The method may include limiting the axial mixing of the solid particlesover the entire reactor length. This can be achieved through theselection of vertically spaced reaction zones and downcomers spanningthe length of a single reaction zone.

According to a third aspect of the invention, there is provided athree-phase slurry reactor, the reactor including

-   a reactor shell containing a plurality of vertically extending    horizontally spaced slurry channels which, in use, will contain a    slurry of solid particles suspended in a suspension liquid, the    slurry channels being defined between vertically extending    horizontally spaced divider walls or plates and each slurry channel    having a height, width and breadth such that the height and breadth    are much larger than the width;-   a gas inlet in the reactor shell for introducing a gaseous reactant    or gaseous reactants into the reactor; and-   a gas outlet in the shell for withdrawing gas from a head space in    the shell above the slurry channels.

At least some of the divider walls or plates may define heat transfermedium flow spaces or channels. The heat transfer medium flow channelsmay also have a height, breadth and width such that the height andbreadth are much larger than the width.

Heat transfer surfaces of the reactor, such as those of the dividerwalls or plates, may optionally be shaped or textured to increase theirheat transfer surface area or to improve heat transfer coefficients. Theshaping or texturing may include, amongst other methods known to personsskilled in the art, the use of dimpled, ribbed or finned walls orplates.

The channels may be as hereinbefore described.

The slurry channels are thus located in a slurry zone inside the reactorshell. The slurry zone may have a normal slurry level above open upperends of the slurry channels so that at least some of the slurry channelsmay be in slurry flow communication above their open upper ends.

The reactor may include one or more downcomer zones or downcomers, inuse through which slurry can pass from a high level in the slurry zoneto a lower level thereof.

According to a fourth aspect of the invention, there is provided athree-phase slurry reactor, the reactor including

-   a reactor shell containing a plurality of vertically extending    horizontally spaced slurry channels which, in use, will contain a    slurry of solid particles suspended in a suspension liquid, the    slurry channels being located in a slurry zone inside the reactor    shell which has a normal slurry level above open upper ends of the    slurry channels so that at least some of the slurry channels are in    slurry flow communication above their open ends;-   a heat transfer medium flow space or spaces defined by walls of the    slurry channels separating the slurry channels from the heat    transfer medium flow space or spaces so that in use heat transfer in    indirect heat transfer relationship can take place between slurry in    the slurry channels and a heat transfer medium in the heat transfer    medium flow space or spaces;-   one or more downcomer zones or downcomers through which slurry can    pass from a high level in the slurry zone to a lower level thereof;-   a gas inlet in the reactor shell for introducing a gaseous reactant    or gaseous reactants into the reactor;-   a gas outlet in the shell for withdrawing gas from a head space in    the shell above the slurry channels; and-   if necessary, a liquid inlet for adding or withdrawing slurry or    suspension liquid to or from the reactor.

At least some of the slurry channels may be in slurry flow communicationbelow open lower ends of the slurry channels. The slurry channels mayhave walls configured to prevent slurry flow from or into the slurrychannels other than through open upper and lower ends of the slurrychannels. In other words, the walls typically prevent radial ortransverse slurry flow between slurry channels, so that the slurrychannels are completely individualised reaction chambers.

The slurry channels in the reactor according to the fourth aspect of theinvention may be defined by vertically extending tubes between tubesheets, with the heat transfer medium flow space being defined betweenthe tubes sheets and surrounding the tubes. The tubes typically havediameters of at least about 10 cm.

Instead, the slurry channels may be defined by vertically extendinghorizontally spaced divider walls or plates, with the heat transfermedium flow spaces also being defined between vertically extendinghorizontally spaced divider walls or plates, at least some slurrychannels being separated from adjacent heat transfer medium flow spacesby common or shared divider walls or plates.

The divider walls or plates may be parallel to each other, definingslurry channels and heat transfer medium flow spaces as hereinbeforedescribed. Typically, the divider walls or plates correspond with chordsof the circular cylindrical reactor shell, when seen in plan view.

When the slurry channels are defined by divider walls, the slurrychannels and heat transfer medium flow spaces may be alternatelyarranged. Each slurry channel may thus be flanked by, or sandwichedbetween, two heat transfer medium flow spaces, except possibly forradially outer slurry channels.

Heat transfer surfaces of the reactor, such as those of the dividerwalls or plates or tubes, may optionally be shaped or textured toincrease their heat transfer surface area or to improve heat transfercoefficients compared to those of smooth divider walls or smoothcylindrical tubes. The shaping or texturing may include, amongst othermethods known to persons skilled in the art, the use of dimpled, ribbedor finned walls or plates or tubes.

The slurry channels, optionally together with one or more heat transfermedium flow spaces, may be grouped together in reactor modules orsub-reactors. Sub-reactors may be horizontally disposed across thecross-sectional area of the reactor shell. A sub-reactor may havevertically extending side walls separating it from an adjacenthorizontally spaced sub-reactor. The vertically extending side wall maybe configured to prevent slurry flow communication between adjacenthorizontally disposed sub-reactors at all elevations between upper andlower open ends of the slurry channels of the adjacent horizontallydisposed sub-reactors.

The slurry channels of horizontally disposed or horizontally spacedadjacent sub-reactors may each have a breadth axis, when the slurrychannels are defined by divider walls or plates, with the breadth axesof the slurry channels of adjacent horizontally disposed sub-reactorsbeing parallel. Instead, the breadth axes of adjacent horizontallydisposed sub-reactors may be perpendicular. In such an embodiment, anend divider wall of a sub-reactor may thus form a side wall separatingthe sub-reactor from a horizontally disposed adjacent sub-reactor.

The reactor may include reactor modules or sub-reactors which arevertically spaced, with the open upper ends of the slurry channels of alower sub-reactor or sub-reactors being below the open lower ends of theslurry channels of an upper sub-reactor or sub-reactors.

The reactor may include an intermediate zone between uppersub-reactor(s) and lower sub-reactor(s). The intermediate zone may be inflow communication with slurry channels of an upper sub-reactor orsub-reactors and with slurry channels of a lower sub-reactor orsub-reactors. In other words, transverse or horizontal flow or mixing ofslurry in the intermediate zone may be allowed by having theintermediate zone free of barriers which would prevent transverse flowbetween open ends of slurry channels opening out into the intermediatezone.

The reactor may include a gas inlet into an intermediate zone betweenupper and lower sub-reactors. The gas inlet may be a recycle gas inlet.The gas inlet may be configured to introduce gas only into a portion ofthe cross-sectional area of the reactor shell. In other words, the gasinlet may be arranged in use to gas only a selected cross-sectionalregion of the reactor, e.g. only certain sub-reactors or certain slurrychannels.

One or more downcomer zones or downcomers may extend from at or abovethe open upper ends of the slurry channels, or the slurry channels of anupper sub-reactor, to at or below open lower ends of the slurrychannels, or slurry channels of a lower sub-reactor.

Instead, one or more of the downcomer zones or downcomers may extendfrom at or above the open upper ends of the slurry channels in asub-reactor, to at or below open lower ends of the slurry channels ofsaid sub-reactor, often into an intermediate zone below saidsub-reactor. Downcomer zones or downcomers of vertically spacedsub-reactors may be staggered in plan view, or may be in register.

A downcomer or downcomer zone may be defined by slurry channels adaptedto function as a downcomer or downcomer zone. Such an adapted slurrychannel may have or may be associated with a gassing prevention device,e.g. a baffle, or it may have or it may be associated with a degasser atan upper end thereof.

A downcomer zone or downcomer may include a heat transfer medium flowspace or spaces and/or it may include a filter to separate solidparticles from suspension liquid.

The heat transfer medium flow spaces, when in the form of channels, areclose-ended, and are provided with heat transfer medium inlet and outletarrangements. The heat transfer medium inlet and outlet arrangements mayopen out into the channels through their closed ends, i.e. axially orvertically, or the heat transfer medium flow channels or spaces may bein flow communication transversely or horizontally, reminiscent of aplate heat exchanger in which every second flow space is in flowcommunication, whilst being sealed from intervening flow spaces.

The invention will now be described, by way of example, with referenceto the accompanying diagrammatic drawings, in which

FIG. 1 shows a schematic sectional elevational view of one embodiment ofa three-phase slurry reactor in accordance with the invention;

FIG. 2 shows a schematic sectional elevational view of anotherembodiment of a three-phase slurry reactor in accordance with theinvention;

FIG. 3 shows a schematic three-dimensional view of some reactor modulesor sub-reactors and downcomers or downcomer zones of a three-phaseslurry reactor in accordance with the invention;

FIG. 4 shows a schematic top plan view of the reactor modules anddowncomers of FIG. 3;

FIG. 5 shows a schematic three-dimensional view of some upper and lowerreactor modules or sub-reactors and downcomers of a three-phase slurryreactor in accordance with the invention;

FIGS. 6 to 9 show schematic sectional elevational views of variousembodiments of three-phase slurry reactors in accordance with theinvention, with or without downcomers;

FIGS. 10 to 12 show schematic sectional elevational views of variousembodiments of three-phase slurry reactors in accordance with theinvention, with stage introduction of gas and various downcomerarrangements;

FIGS. 13 to 16 show schematic top plan views of various arrangements ofdivider walls of three-phase slurry reactors in accordance with theinvention;

FIGS. 17 to 20 show schematic sectional plan views of variousthree-phase slurry reactors in accordance with the invention,illustrating various downcomer arrangements; and

FIGS. 21 to 28 show schematic sectional plan views of variousthree-phase slurry reactors in accordance with the invention,illustrating various arrangements of horizontally disposed reactormodules or sub-reactors and downcomer zones.

Referring to FIG. 1 of the drawings, reference numeral 10 generallyindicates one embodiment of a three-phase slurry reactor in accordancewith the invention. The reactor 10 includes a reactor shell 12 whichhouses a plurality of vertically extending, horizontally spaced paralleldivider walls or plates 14. The plates 14 define a plurality of slurrychannels 16.

The shell 12 is circular cylindrical and the plates 14 correspond withor fall on chords of the shell 12, when viewed in plan. Each slurrychannel 16 has a relatively small width, i.e. the spacing between theplates 14, compared to its height and its breadth, where its breadth istaken along an axis perpendicular to the page on which the drawing isshown.

Although not shown in the drawings, at least some of the divider wallsor plates 14 may be shaped or textured to increase their heat transfersurface area or to improve heat transfer coefficients. The shaping ortexturing may include, amongst other methods known to persons skilled inthe art, the use of dimpled, ribbed or finned walls or plates.

The reactor 10 also includes a gas inlet 18 leading into a spargerarrangement 20 below the slurry channels 16. A gas outlet 22 is providedwhich is in flow communication with a head space 24 above the slurrychannels 16. A liquid outlet 26 leads from a bottom of the reactor 10,below the slurry channels 16, but can be at any convenient level.

The reactor 10 has a slurry zone extending from the bottom of thereactor 10 to a normal slurry level indicated by reference numerals 28and 30. As can be seen in FIG. 1, the normal slurry level 28 can thuseither be below the open upper ends of the slurry channels 16, or thenormal slurry level 30 may be above the open upper ends of the slurrychannels 16, thereby in use completely submerging the plates 14.

In a slurry reactor such as the reactor 10, there would be limited orsubstantially no interaction between the slurry channels 16 where theyopen out into the bottom of the reactor 10. Reaction spaces, defined bythe slurry channels 16, are essentially two-dimensional and if theslurry channels are operated essentially independent of each other thedependency upon the diameter of the reactor shell 12 largely orcompletely disappears. This facilitates scale-up, as a representativeunit, consisting of one or a few slurry channels, can be studiedseparately and independently from commercial scale reactor dimensions.

When the plates 14 are not fully submerged in the slurry body, i.e. whenthe normal slurry level is the level 28, the reactor 10 essentiallybehaves as a stack of parallel, vertically extending two-dimensionalthree-phase slurry columns. Differences between these two-dimensionalcolumns and conventional three-dimensional columns, relating to mixing,gas hold-up and heat and mass transfer, may be used advantageously.

For fully submerged plates 14, when the normal slurry level is indicatedby the level 30, even more opportunities present themselves. A slurrycirculation flow pattern over the slurry channels 16 can be established,allowing for better plug flow characteristics for the phases in theslurry channels 16, a more uniform solids distribution throughout theslurry and higher heat transfer coefficients (reactors with heattransfer arrangements will be discussed in more detail later on).

Referring to FIG. 2 of the drawings, reference numeral 100 generallyindicates another embodiment of a three-phase slurry reactor inaccordance with the invention. The reactor 100 is similar to the reactor10 in many respects and the same reference numerals are thus used toindicate the same or similar parts or features, unless otherwiseindicated. In the reactor 100, heat transfer medium channels 32 are alsodefined between some of the plates 14. The heat transfer medium channels32 have closed lower ends and upper ends, but are in flow communicationwith each other at their ends and with heat transfer medium inlet andoutlet arrangements (not shown). In use, heat transfer medium can thusbe passed through the heat transfer medium channels 32, either upwardlyor downwardly.

The slurry channels 16 and the heat transfer medium channels 32 arearranged alternately, so that each slurry channel 16 is flanked by orsandwiched between two heat transfer medium channels 32, except possiblyfor radially outer slurry channels 16, depending on the particularconstruction of the reactor 100.

In the reactor 100, the slurry channels 16 and the heat transfer mediumchannels 32 are grouped into an upper group, defining an upper platebank or sub-reactor 34 and a lower group defining a lower plate bank orsub-reactor 36. The upper sub-reactor 34 is vertically spaced from thelower sub-reactor 36 so that the open lower ends of the slurry channels16 of the upper sub-reactor 34 are above the open upper ends of theslurry channels 16 of the lower sub-reactor 36. Between the uppersub-reactor 34 and the lower sub-reactor 36 an intermediate zone 38 isdefined. A gas inlet, which is a recycle gas inlet and which isindicated by reference numeral 40 enters the intermediate zone 38 fromtwo diagonally opposed sides of the reactor 100. Each recycle gas inlet40 is associated with a sparger arrangement 42.

A downcomer 44 with a degasser 46 is provided centrally in the reactorshell 12 and extends from above the open upper ends of the slurrychannel 16 of the upper sub-reactor 34 to below the open lower ends ofthe slurry channels 16 of the upper sub-reactor 34, i.e. into theintermediate zone 38. Between the reactor shell 12 and the plates 14 ofthe lower sub-reactor 36, an annular downcomer zone 48 is defined. Aswill be noticed, the sparger arrangements 42 are configured not to gasthe downcomer 44 and the sparger arrangement 18 is configured not to gasthe downcomer zone 48. As will be appreciated, the downcomer 44 is ineffect staggered relative to the downcomer zone 48, ensuring a slurryrecycle or redistribution flow as indicated by the arrows 50.

The reactor 100 in principle is suitable for many processes requiring athree-phase slurry reactor and requiring heat transfer to or from theslurry. However, only one use, namely hydrocarbon synthesis, will now bedescribed.

In use, fresh synthesis gas comprising mainly carbon monoxide andhydrogen as gaseous reactants, is fed into the bottom of the reactor 100through the gas inlet 18 and the sparger arrangement 20. By means of thesparger arrangement 20, the synthesis gas is uniformly distributedthroughout the slurry present in the bottom of the reactor 100.Simultaneously, a recycle gas stream (typically cooled) comprisingtypically hydrogen, carbon monoxide, methane and carbon dioxide isreturned to the reactor 100 through the recycle gas inlets 40 and thesparger arrangements 42. All of the recycle gas stream may be fed intothe intermediate zone 38 by means of the recycle gas inlets 40 or, ifdesired, a portion of the recycle gas stream may be returned to thebottom of the reactor 100, by means of the gas inlet 18.

By means of the sparger arrangements 42, the slurry channels 16 of theupper sub-reactor 34 are specifically targeted with recycle gas, and thedowncomer 44 is avoided. By using the recycle gas inlets 40, it is thuspossible to allow a portion of the recycle gas to bypass the slurrylocated in the portion of the reactor 100 below the sparger arrangements42. In this fashion, the overall gas hold-up in the reactor 100 can bereduced, thereby surprisingly increasing the reactor capacity.

The gaseous reactants, comprising the fresh synthesis gas and anyrecycle gas, pass upwardly through a slurry body 52 which occupies theslurry channels 16 of the upper and lower sub-reactors 34, 36 and whichextends from the bottom of the reactor 100 to the level 30. The slurrybody 52 comprises Fischer-Tropsch catalyst particles, typically an iron-or cobalt-based catalyst, suspended in liquid product (mostly wax). Theslurry body 52 is controlled to have the slurry level 30 above the openupper ends of the slurry channels 16 of the upper sub-reactor 34 andabove the degasser 46.

As the synthesis gas bubbles through the slurry body 52, the gaseousreactants therein react catalytically and exothermically to form liquidproduct, which thus forms part of the slurry body 52. From time to time,or continuously, slurry or liquid phase including liquid product iswithdrawn through the liquid outlet 26, with the slurry level 30 therebybeing controlled. The catalyst particles are separated from the liquidproduct in a suitable internal or external separation system, e.g. usingfilters (not shown). If the separation system is located externally tothe reactor 100, an additional system (not shown) to return theseparated catalyst particles to the reactor 100 is then provided.

The fresh synthesis feed gas and the recycle gas are introduced into thereactor 100 at a rate sufficient to agitate and suspend all of thecatalyst particles inside the reactor 100 without settling. The gas flowrate will be selected depending on the slurry concentration, catalystdensity, suspending medium density and viscosity, and particularparticle size used. Suitable gas flow rates include, for example, fromabout 5 cm/s to about 50 cm/s. However, gas velocities up to about 85cm/s have been tested in bubble columns. The use of higher velocitieshas the disadvantage that it is accompanied by a higher gas hold-up inthe reactor leaving relatively less space to accommodate thecatalyst-containing slurry. Whatever gas flow rate is however selected,it should be sufficient to avoid particle settling and agglomeration inthe reactor 100.

Some slurry continuously passes downwardly through the downcomer 44 andthe downcomer zone 48 as indicated by the arrows 50, thereby to achieveredistribution of catalyst particles within the slurry body 52 and topromote uniform heat redistribution throughout the slurry body 52. Aswill be appreciated, depending on the arrangement of the downcomers ordowncomer zones, slurry redistribution over selected verticallyextending regions of the reactor 100 is possible.

The reactor 100 is operated such that the slurry body 52 in the slurrychannels 16 is in a heterogeneous or churn-turbulent flow regime andcomprises a dilute phase consisting of fast-rising larger bubbles ofgaseous reactants and gaseous product which traverse the slurry body 52virtually in plug flow fashion and a dense phase which comprises liquidproduct, solid catalyst particles and entrained smaller bubbles ofgaseous reactants and gaseous product. By means of the use of the slurrychannels 16, the plug flow behaviour of the entire reactor 100 ispromoted, since each slurry channel 16 has a high aspect ratio whenheight and width are considered, which is well in excess of the aspectratio of the reactor shell 12.

Preferably, the downflow rate of slurry in downcomer zones 44 and 48 issufficiently high, that there is substantially no downward flow ofslurry in the slurry channels 16. In this manner, the establishment of amacro-mixing pattern other than downward in the downcomer zones 44 and48 and upwards in the slurry channels 16 is largely precluded.

The slurry body 52 is present in alternate, open-ended, slurry channels16 in the upper sub-reactor 34 and the lower sub-reactor 36. Boiler feedwater as cooling medium is circulated through the closed-ended heattransfer medium channels 32 to remove the heat of the exothermicreactions. As will be appreciated, the plates 14 provide large heattransfer surface areas for removing heat from the slurry body 52 bymeans of indirect heat transfer to the boiler feed water.

Light hydrocarbon products, such as a C₂₀ and below fraction arewithdrawn from the reactor 100 through the gas outlet 22 and passed to aseparation unit (not shown). Typically, the separation unit comprises aseries of coolers and a vapour-liquid separator and may optionallyinclude further coolers and separators and possibly also a cryogenicunit for removal of hydrogen, carbon monoxide, methane and carbondioxide from the C₂₀ and below hydrocarbon fraction. Other separationtechnologies such as membrane units, pressure swing adsorption unitsand/or units for the selective removal of carbon dioxide may beemployed. The separated gases comprising nitrogen, carbon monoxide andother gases are compressed and recycled by means of a compressor (notshown) to provide the recycle gas stream. Condensed liquid hydrocarbonsand reaction water are withdrawn from the separation unit for furtherworking-up.

It is to be appreciated that, although the reactor 100, as illustrated,allows for the recycle of gas to the reactor 100, it is not necessarilyso that a recycle gas stream will be employed in all embodiments.

As a result of the presence of the plates 14, no slurry flowcommunication is possible between the slurry channels 16, at allelevations between their open upper ends and their open lower ends.However, above the open upper ends of the slurry channels 16 of theupper sub-reactor 34, there is no restriction on the flow of slurry.Similarly, in the intermediate zone 38 and below the open lower ends ofthe slurry channels 16 of the lower sub-reactor 36 there is norestriction on the flow of slurry.

A three-phase slurry reactor in accordance with the invention mayinclude a plurality of horizontally disposed reactor modules orsub-reactors, which will thus be at the same elevation inside thereactor shell 12 but disposed across the cross-sectional area of thereactor shell 12. In FIGS. 3 and 4, a few of these horizontally disposedreactor modules or sub-reactors or plate banks are shown and indicatedby reference numeral 60. Associated with the sub-reactors 60, aredowncomer zones indicated by reference numeral 62. A sparger arrangement64 is provided below the sub-reactors 60 and downcomer zones 62.

As will be noted, the downcomer zones 62 also include a plurality ofvertically extending divider walls or plates 14 in the same fashion asthe sub-reactors 60. However, the sparger arrangement 64 does not gasthe downcomer zones 62, allowing the zones 62 to function as downcomersand not as sub-reactors or risers.

Like the sub-reactors 60, the downcomer zones 62 have slurry channelsand heat transfer medium channels which are alternately arranged.

In FIGS. 3 and 4, the height of the sub-reactors 60 and the downcomerzones 62 are shown as being equal. It is however to be appreciated thatthey can be different in height, width and channel breadth.

As indicated by the crossed arrows 61 in FIG. 4, there is no slurryexchange between the sub-reactors 60 or between the sub-reactors 60 andthe downcomer zones 62, except above the open upper ends of the slurrychannels and below the lower open ends of the slurry channels.

The parallel plates of a sub-reactor or plate bank may define channels16 with open sides, as shown in FIG. 13, or the sub-reactors may haveside walls 63 as shown in FIG. 14. When the sides of the channels 16 areclosed by side walls 63, as shown in FIG. 14, there can be nointeraction between the slurry in the channels 16 of one suchsub-reactor with the slurry in the channels 16 of an adjacentsub-reactor, unless apertures are provided in the side walls 63.Naturally, side walls may enclose more than one sub-reactor or platebank.

When two sub-reactors are arranged with their plates 14 parallel, asshown in FIG. 15, and in the absence of side walls, slurry in thechannels 16 of one sub-reactor can interact with the slurry in thechannels 16 of the adjacent sub-reactor. When the plates 14 of adjacentsub-reactors are perpendicular, as shown in FIG. 16, the end plate ofone sub-reactor in effect defines a side wall, preventing interactionbetween slurry in the channels 16 of the two sub-reactors.

Referring to FIG. 5 of the drawings, upper sub-reactors 34 and lowersub-reactors 36 as well as two downcomers or downcomer zones 62 areshown. Two sparger arrangements 64, one below the upper sub-reactors 34and one below the lower sub-reactors 36, are also shown. In the reactorlayout shown in FIG. 5, the downcomers or downcomer zones 62 extend fromthe upper open ends of the slurry channels of the upper sub-reactors 34through the intermediate zone 38 to below the open lower ends of thelower sub-reactors 36 and in fact to below the lower sparger arrangement64. With this arrangement, large scale axial circulation of slurry in aknown and controlled pattern can be achieved. It is also possible toallow for limited slurry exchange between adjacent sub-reactors 34.a and34.b or 36.a and 36.b. As will be appreciated, the slurry channels canbe designed to have a desired heat transfer surface area, hydraulicdiameter, etc. If desired, additional gas sparging can be installed inbetween vertically spaced sub-reactors, in the intermediate zone 38 andinternal filtration devices can be installed in the intermediate zone 38or in one of the downcomers or downcomer zones 62. One advantage ofplacing internals such as filters in a downcomer or downcomer zone isthe reduced gas hold-up and relatively high velocities encountered in adowncomer zone. By selecting the locations of the downcomers ordowncomer zones 62 and placing them in particular positions on thecross-sectional area of the reactor shell 12, large scale slurrycirculation can be severely influenced to achieve desired objects.

Downcomers or downcomer zones can be helpful in levelling the solidshold-up profile and temperature profile over the height of a three-phaseslurry reactor. At the same time, however, they induce axial mixing,which sometimes may not be desirable. By design, the axial mixing can bepromoted (resulting in a kind of riser-downcomer mode of operation) orit can be suppressed in order to promote plug flow characteristics forthe reactor.

FIGS. 6 to 9 show various embodiments of three-phase slurry reactors inaccordance with the invention, with various downcomer arrangements. InFIG. 6, the reactor has four vertically spaced sub-reactors or platebanks, with no downcomer. In FIG. 3, it is shown that a top to bottomdowncomer, extending linearly axially through the sub-reactors or platebanks, can be employed. FIG. 8 illustrates how downcomers in eachsub-reactor or plate bank can be arranged so that the downcomers, whenviewed in plan, are staggered between upper and lower sub-reactors orplate banks. FIG. 9 illustrates a three-phase slurry reactor withdivider walls or plates extending substantially the entire length of thereactor, from a bottom region to a head space region, with a singledowncomer extending from the head space to the bottom region.

Various arrangements of downcomers or downcomer zones are shown in FIGS.17 to 20 in which the downcomers or downcomer zones are indicated byreference numeral 70. In FIG. 17, the downcomer zones 70 are distributedacross the cross-sectional area of the reactor shell 12. In FIG. 18, thedowncomer zone 70 is adjacent the reactor shell 12, being roughlyannular in plan view. The downcomer zone 70 in FIG. 19 is against oneside of the reactor shell 12 and in FIG. 20, the downcomer zone 70 iscentrally located.

By means of the arrangement selected for the sub-reactors and downcomeror downcomer zones, it is possible to allow or prevent slurry flowinteraction between different upflow zones in the reactor (defined bythe slurry channels), and to prevent or deny interaction between theseupflow zones and downflow zones (defined by the downcomers or downcomerzones). Thus, in a reactor such as the reactor 100, at a particularelevation such as the elevation indicated by reference numeral 72 inFIG. 2, many configurations are possible, some of which are shown inFIGS. 21 to 28 of the drawings.

In FIG. 21, the downflow zones 70 are against the sides of the reactorshell 12. Each sub-reactor, indicated by reference numeral 74 has sidewalls, thereby preventing slurry interaction between the sub-reactors74, and between the sub-reactors 74 and the downflow zones 70.

The sub-reactors 74 in FIG. 22 do not have side walls and the slurrychannels of adjacent sub-reactors 74 are parallel. The slurry in theseslurry channels can thus interact. In contrast, in FIG. 23, the slurrychannels of adjacent sub-reactors 74 are arranged perpendicularly. Theindividual sub-reactors 74 do not have side walls, although the group oftwenty-five sub-reactors has a side wall 76. The sub-reactors 74 arespaced slightly, allowing limited slurry interaction between adjacentsub-reactors 74 but with the perpendicular arrangement of the platespreventing a more free slurry interaction between adjacent sub-reactors74. No slurry interaction is allowed between the upflow zones, i.e. thesub-reactors 74 and the downcomer zones 70.

In FIG. 24, the sub-reactors 74 are all provided with side walls and thedowncomer zones 70 are distributed. There is thus no slurry interactionbetween the sub-reactors 74, or between the sub-reactors 74 and thedowncomer zones 70. In contrast, in FIG. 25, the sub-reactors 74 do nothave side walls and the downcomer zones 70 are only adjacent the shell12. Substantial slurry interaction between the sub-reactors 74, andbetween the sub-reactors 74 and the downcomer zones 70 can take place.In FIG. 26, the sub-reactors 74 are again without side walls, but manyare arranged with their slurry channels perpendicular to the slurrychannels of adjacent sub-reactors 74. Although there will thus be someinteraction between adjacent sub-reactors 74 and between thesub-reactors 74 and the downcomer zones 70, the slurry interaction willbe more limited than in the case of the reactor shown in FIG. 25.

FIG. 27 shows a reactor similar to the reactor shown in FIG. 26, but inthe case of the reactor of FIG. 27, the downcomer zones 70 are disposedacross the cross-sectional area of the reactor.

In FIG. 28, the downcomer zone 70 is located against one side of thereactor shell 12. There is limited slurry interaction between thesub-reactors 74 as a result of the slight spacing between thesub-reactors 74, although they are arranged at perpendicular angles. Abarrier or side wall 76 substantially prevents slurry interactionbetween the slurry in the sub-reactors 74 and the slurry in thedowncomer zone 70.

Various gas sparging strategies are shown in FIGS. 10 to 12. In FIG. 10,the gas is introduced in two stages, a portion of the gas entering abottom region of the reactor and another portion of the gas entering anintermediate zone between two sub-reactors or plate banks. In FIGS. 11and 12, the gas spargers are shown in combination with downcomers ordowncomer zones. As can be clearly seen, it is possible to gas only aportion of the cross-sectional area of the reactor, in both the bottomand in the intermediate zones.

The method and apparatus of the present invention therefore allow formuch reduced risk when upscaling slurry flow reactor designs, since theformation of macro-scale mixing patterns are largely prevented by thepresence of slurry channels. In addition, and especially for designsincluding downcomers or downcomer zones, the reaction zone consists of anumber of slurry channels in which a known upward superficial liquidflow rate and a known upward superficial gas velocity exist. Theseslurry channels are amenable to piloting and modelling, giving thedesigner a greater degree of control over the large scale reactor mixingpatterns. Furthermore, the slurry channels are formed by heat exchangersurfaces. This leads to much improved heat removal ability for thesedesigns over standard designs in which serpentine cooling coils areemployed. Not only is the available heat removal surface area increased,but also more uniformly spread over the reactor.

1.-32. (canceled)
 33. A three-phase-slurry reactor, the reactorincluding a reactor shell containing a plurality of vertically extendinghorizontally spaced open-ended slurry channels which, in use, willcontain a slurry of solid particles suspended in a suspension liquid,the slurry channels being defined between vertically extendinghorizontally spaced divider walls or plates and each slurry channelhaving a height, width and breadth such that the height and breadth aremuch larger than the width, the slurry channels being grouped togetherin reactor modules or sub-reactors; a gas inlet in the reactor shell forintroducing a gaseous reactant or gaseous reactants into the reactor;and a gas outlet in the shell for withdrawing gas from a head space inthe shell above the slurry channels.
 34. The reactor as claimed in claim33, in which at least some of the divider walls or plates at leastpartially define heat transfer medium flow spaces or channels.
 35. Thereactor as claimed in claim 33, in which the slurry zone has a normalslurry level above open upper ends of the slurry channels so that atleast some of the slurry channels are in slurry flow communication abovetheir open upper ends.
 36. The reactor as claimed in claim 33, in whichthe reactor modules or sub-reactors are horizontally disposed across thecross-sectional area of the reactor shell.
 37. The reactor as claimed inclaim 36, in which the sub-reactors have vertically extending side wallsseparating them from adjacent horizontally spaced sub-reactors and inwhich the vertically extending side walls are configured to preventslurry flow communication between adjacent horizontally spacedsub-reactors at all elevations between upper and lower open ends of theslurry channels of the adjacent horizontally disposed sub-reactors. 38.The reactor as claimed in claim 36, in which the slurry channels aredefined by divider walls or plates that are parallel within eachsub-reactor so that the adjacent sub-reactors each have a breadth axis,and in which the breadth axes of adjacent horizontally disposedsub-reactors are perpendicular.
 39. The reactor as claimed in claim 36,which includes reactor modules or sub-reactors which are verticallyspaced, with the open upper ends of the slurry channels of a lowersub-reactor or sub-reactors being below the open lower ends of theslurry channels of an upper sub-reactor or sub-reactors.
 40. The reactoras claimed in claim 39, which includes an intermediate zone betweenupper sub-reactor(s) and lower sub-reactor(s), with said intermediatezone being in flow communication with slurry channels of an uppersub-reactor or sub-reactors and with slurry channels of a lowersub-reactor or sub-reactors.
 41. The reactor as claimed in claim 40,which includes a gas inlet into said intermediate zone between upper andlower sub-reactors.
 42. The reactor as claimed in claim 33, in which oneor more downcomer zones or downcomers extend from at or above the openupper ends of the slurry channels, or the slurry channels of an uppersub-reactor if present, to at or below open lower ends of the slurrychannels, or slurry channels of a lower sub-reactor if present, and/orin which one or more downcomer zones or downcomers extend from at orabove the open upper ends of the slurry channels in a sub-reactor, to ator below open lower ends of the slurry channels of said sub-reactor. 43.A three-phase slurry reactor, the reactor including a reactor shellcontaining a plurality of vertically extending horizontally spacedslurry channels which, in use, will contain a slurry of solid particlessuspended in a suspension liquid, the slurry channels being located in aslurry zone inside the reactor shell which has a normal slurry levelabove open upper ends of the slurry channels so that at least some ofthe slurry channels are in slurry flow communication above their openends, the slurry channels being grouped together in reactor modules orsub-reactors; a heat transfer medium flow space or spaces defined bywalls of the slurry channels separating the slurry channels from theheat transfer medium flow space or spaces so that in use heat transferin indirect heat transfer relationship can take place between slurry inthe slurry channels and a heat transfer medium in the heat transfermedium flow space or spaces; one or more downcomer zones or downcomersthrough which slurry can pass from a high level in the slurry zone to alower level thereof; a gas inlet in the reactor shell for introducing agaseous reactant or gaseous reactants into the reactor; a gas outlet inthe shell for withdrawing gas from a head space in the shell above theslurry channels; and if necessary, a liquid inlet for adding orwithdrawing slurry or suspension liquid to or from the reactor.
 44. Thereactor as claimed in claim 43, in which at least some of the slurrychannels are in slurry flow communication below open lower ends of theslurry channels, the slurry channels having walls configured to preventslurry flow from or into the slurry channels other than through openupper and open lower ends of the slurry channels.
 45. The reactor asclaimed in claim 43, in which the slurry channels in the reactor aredefined by vertically extending tubes between tube sheets, with the heattransfer medium flow space being defined between the tubes sheets andsurrounding the tubes.
 46. The reactor as claimed in claim 43, in whichthe slurry channels are defined by vertically extending horizontallyspaced divider walls or plates, with the heat transfer medium flowspaces also being defined between vertically extending horizontallyspaced divider walls or plates, and at least some of the divider wallsor plates being parallel to each other, defining slurry channels andheat transfer medium flow spaces with a height, width and breadth suchthat the height and breadth are much larger than the width.
 47. Thereactor as claimed in claim 43, in which the reactor modules orsub-reactors are horizontally disposed across the cross-sectional areaof the reactor shell.
 48. The reactor as claimed in claim 47, in whichthe sub-reactors have vertically extending side walls separating themfrom adjacent horizontally spaced sub-reactors and in which thevertically extending side walls are configured to prevent slurry flowcommunication between adjacent horizontally spaced sub-reactors at allelevations between upper and lower open ends of the slurry channels ofthe adjacent horizontally disposed sub-reactors.
 49. The reactor asclaimed in claim 47, in which the slurry channels are defined by dividerwalls or plates that are parallel within each sub-reactor so that theadjacent sub-reactors each have a breadth axis, and in which the breadthaxes of adjacent horizontally disposed sub-reactors are perpendicular.50. The reactor as claimed in claim 47, which includes reactor modulesor sub-reactors which are vertically spaced, with the open upper ends ofthe slurry channels of a lower sub-reactor or sub-reactors being belowthe open lower ends of the slurry channels of an upper sub-reactor orsub-reactors.
 51. The reactor as claimed in claim 50, which includes anintermediate zone between upper sub-reactor(s) and lower sub-reactor(s),with said intermediate zone being in flow communication with slurrychannels of an upper sub-reactor or sub-reactors and with slurrychannels of a lower sub-reactor or sub-reactors.
 52. The reactor asclaimed in claim 51, which includes a gas inlet into said intermediatezone between upper and lower sub-reactors.
 53. The reactor as claimed inclaim 43, in which one or more downcomer zones or downcomers extend fromat or above the open upper ends of the slurry channels, or the slurrychannels of an upper sub-reactor if present, to at or below open lowerends of the slurry channels, or slurry channels of a lower sub-reactorif present, and/or in which one or more downcomer zones or downcomersextend from at or above the open upper ends of the slurry channels in asub-reactor, to at or below open lower ends of the slurry channels ofsaid sub-reactor.